The present invention relates to an air-fuel ratio control system for an internal combustion engine which controls the air-fuel ratio of air-fuel mixture to an approximate value to the stoichiometric air-fuel ratio at which three-way catalyst acts most effectively and more particularly to an air-fuel ratio control system which is capable of reducing the CO content in exhaust gases at low engine temperature.
In a conventional air-fuel ratio control system, the air-fuel ratio of air-fuel mixture burned in cylinders of an engine is detected as an oxygen concentration of the exhaust gases by means of an O.sub.2 -sensor provided in the exhaust system of the engine, and judgement is made by the output signal from the O.sub.2 -sensor as to whether the signal is greater or smaller than the value corresponding to the stoichiometric air-fuel ratio, whereby electromagnetic valves for regulating the air to be mixed with the mixture are opened or closed, and accordingly the air-fuel ratio is controlled to the stoichiometric air-fuel ratio. In such an air-fuel ratio control system, while the throttle valve of the engine is fully opened at low engine temperature, the air-fuel mixture is enriched in order to improve driveability of the vehicle powered by the engine.
The conventional air-fuel ratio control system will be explained with reference to FIG. 1 which illustrates a schematic view of the system. The output from an O.sub.2 -sensor 25 for detecting oxygen concentration in the exhaust gases is applied to a feedback control circuit 26, the output of which is applied to an electromagnetic valve 27 for controlling the air feed rate to a carburetor, thus constituting a feedback control. Further, a negative pressure switch 28 which is turned on in accordance with a predetermined negative pressure in the induction passage of the engine is connected to a water temperature switch 29 which detects the temperature of cooling water and the output of the water temperature switch 29 is connected to the control circuit 26.
In this system, when the temperature of the engine cooling water is lower than a predetermined temperature, and also when the throttle valve is fully opened, which causes low negative pressure in the induction pipe (i.e. the area A in FIG. 2), the feedback control by the O.sub.2 -sensor 25 is cut out and a fixed rich air-fuel mixture is supplied to the engine. In the other operating condition (i.e. the area B in FIG. 2), the air-fuel ratio is controlled in dependency on the detected air-fuel ratio by the O.sub.2 -sensor 25. As shown in FIG. 3 illustrating the variation of the duty ratio under the conditions shown in FIG. 2, the duty ratio is certainly kept at a particular value under the condition of area A. However, in such a conventional system, because the duty ratio is kept at a particular value as long as the condition is in the area A, even if the engine is warmed up, an excessively rich air-fuel mixture is supplied, which will cause a large amount of CO discharge.